How Ornithopters Fly

Flagge flag vlag pavillon

Articulated flapping wings

Description of flapping wing constructions
which have been developed together with the EV-models

1. Requirements of the flapping wing

With large flapping wings it is too costly in terms of labour to construct them according to the method of trial and error. It is much better to limit the many error possibilities by calculations at least somewhat (e.g. with the calculation tool Orni 1). The basics for the construction of a flapping wing are then, in addition to its geometric data, the desired lift distributions during a flapping period. This results in the forces occurring and the required torsion of the wing or the course of the angles of incidence along the half span.

Here, for example, the relevant, extensively optimised functional distributions for a gently inclined flight of a rectangular flapping wing with the all-round airfoil CLARK-Y are shown (principles please see Handbook). In this case, the angle of incidence at the wing root remains constant during the whole flapping period. The distributions of the downwash angle along the span are straight-lined in all three cases (please look at the diagram downwash distributions on the wing).

The weight of the flapping wing should of course be small. The same applies to the moments of inertia around the axis of the flapping wing. This plays an important role during acceleration in the end positions.

There are recent findings about flying with relative constant lift resp. with wing rotation during upstroke. If one is concerned with the construction of a flapping wing, one should also get to know the advantages and disadvantages of this way of flying. In particular, the long distance flight can be improved by the wing rotation (please see Flapping wings with and without wing rotation novelty in German, PDF 0.7 MB).


2. Aeroelastic controlled articulated flapping wing

Basic spar framework of a flapping wing for profiles with known profile polares
with an articulation for an additional flap motion of the hand wing spar. This is pulled down by a spring (spring device here not shown, please see Articulated flapping wings, in German, PDF 1.3MB).

The small, on aerodynamic forces dependent and thereby aeroelastic stroke motion of the hand wing will be used by levers (brown for arm wing, green for hand wing) to control the twisting along the whole wing. In this way an articulated flapping wing with aeroelastic controlled twisting has been developed.

With the exception of the wing root rib all ribs are put freely rotatable on the spars. For the covering of the flapping wing the highly elastic polyurethane film Platilon U 04 is planed (please look at related link 1).

The mode of operation of this articulated flapping wing, with its wing twisting by wing bending against the stroke direction, resembles a little to that of a bird's wing. In the case of the birds however, the rotating or twisting movement of the hand wing is mechanically coupled with its pivoting motion to the rear (please take a look at the book literature Anatomie und Flugbiologie der Vögel by K. Herzog and the paper Lift during wing upstroke, version 10.1, PDF 1.0 MB).


3. With adjustable twisting moment


4. With increasing wing twisting near the wing tip


5. Aeroelastic controlled articulated flapping wing
with adjustable twisting moment and expanding wing twisting near the wing tip

Wing framework of the model EV8 (2004), designed as an aeroelastic controlled articulated flapping wing, combined with a stroke amplitude expanding auxiliary spar of the hand wing at the wing tip section.

The downstroke twisting here corresponds with the theoretical guidelines almost to the wing tip (please also look at the picture which shows the wing structure in upstroke twisting position of the EV8). But still disturbing is the high mass moment of inertia of this flapping wing round its axis of flapping and twisting.

downstroke twisting upstroke twisting
downstroke twisting upstroke twisting

Maximum intended wing twisting (click on picture)

In flight practice the articulated flapping wing has a big advantage. The bending of the hand wing in comparison to the arm wing depends on lift of the hand wing. At the same time, it determines the distribution of the angle of incidence along the wing span. If the amplitude of the bending is estimable on flight pictures, the lift forces of the hand wing in comparison to the gliding flight can be estimated. Furthermore, the distribution of the angle of incidence in the moment of the picture was taken can easily be suggested (please look at EV6 and EV7). With these two information's selective adjustments of the twisting moment of the flapping wing, the driving power and the ratio of the cycle times are possible. Especially flight pictures taken approximately in the middle of up- and downstroke are informative.


In this paper you can find some more details about the
Articulated flapping wings (in German, PDF 1.3MB).

Information´s and suggestions for a further development you will also find in the article Lift during wing upstroke, version 10.1, (PDF 1.0 MB).


6. Covering of flapping wings

Here is the description of the manufacturing processes of the

Covering of a flapping wing with an elastic film

(in German, PDF 360 KB)


7. Wrist for a strong passive bending of the hand wing with support of the lift displacement

The displacement of lift along the wing which is required for flapping flight takes place mainly in the initial and final phase of wing upstroke (please see distances between the centres of lift). It is advantageous to support it by a sequence of suitable changes in the angle of attack of the various wing sections. A good way to do this is to bend the hand wing downwards. The constructional effort for a strong bending of the hand wing however, is only worthwhile, if a high climbing performance is aimed for the model. In this case, the difference in lift between the arm and hand wings is particularly large during upstroke. As much lift as possible must be concentrated in the middle of the span. The bending of the hand wings with their end plate effect helps here. But even without this functional background, the bending in flight of an ornithopter looks good.

To put it somewhat simply, the bending of the hand wing begins and ends in birds with the beginning and end of the upstroke (please see Lift during wing upstroke, version 10.1, PDF 1.0 MB ).

Wrist for strong bend of the hand wing
Concept for a wing wrist

If one inclines the axis of the hand wing from the back top to the bottom front, an increasing angle of bending is combined with an automatic increase in the angle of incidence (please see adjacent drawing). In this way, the angle of incidence changes self-acting in the desired direction at the beginning and end of the upstroke. This influence on the angle of incidence can approximately replace or at least complement the twisting of the hand wing during upstroke. In addition, the tip of the hand wing is slightly swinging backwards, although not so far as in the birds (tip-reversal upstroke). But they may use a similar operating mode of their wrists.

During the bending of the hand wing, its angle of incidence or angle of attack increases. Therefore, the bending comes to a standstill already after a short upstroke motion. Depending on the upstroke speed, this is the case with a different bending. In this way, the maximum size of the bending automatically adapts to the beat frequency. The higher the beat frequency, the greater is the bending.

hand wing axis pivoted inwards
Pivot angle κ (Kappa) of the wrist axis for an automatic increase of the angle of incidence of the hand wing by the bending

Due to an additional pivoting of the wrist axis at the rear inwards, towards to the span centre (see picture aside), the influence of the bending on the angle of incidence increases. This influence is reversed if the wrist axis is pivoted back outwards. In this way, if necessary, the angle of inclination of the axis on front to bottom can be made larger and thus also the swing of the flapping wing tip to the rear. Both axis directions can complement or replace each other. However, they have different effects on the wing surface in the wrist area. These axis inclinations can also be applied to simple membrane flapping wings.

With the articulated flapping wings used so far, it has proven to be advantageous, also to control the twisting of the arm wing by the passive bending of the hand wing. In the above drawing, therefore, the fuselage near part of the wrist was approximately taken over from the above aeroelastic controlled articulated flapping wing (according to literature Karl Herzog, the wrist of birds is also divided into two partial joints, a proximal ulnacarpal joint and a distal mediocarpal joint). The necessary twisting of the arm wing during up and down stroke is thereby already completely carried out during a small bending of the hand wing, in the close range of the stretched wing position (please see Pictures of the flying EV7a). If the hand wing according the drawing of the wrist bends further, the twisting of the arm wing remains unchanged. This behaviour of the arm wing has advantages in displacement of lift.

The motion of the spring rod should be blocked in the upper end position. Otherwise the moment of inertia of the hand wing will cause an overshoot of the angle of attack, like in the model EV7a.


7.1 Motion sequence of arm and hand wing

When reaches the wing the lower final stroke position, decrease the lift force in the outer wing section. The spring rod increases the angle of attack of the arm wing, at least close to the wrist. In this way, the arm wing takes over the lift of the hand wing. In the lower final stroke position, thus there is a strong lift for a short time. But also, the angle of attack of the hand wing has increased during this time.

Also, with birds, not only the angle of attack of the arm wing, but with it also that of the hand wing is increased with rotation of the wing root in the lower final stroke position. But only with a rotation of the flapping wing the arm wing alone can absorb the whole lift for the upstroke. And only with a large lift of the arm wing are fully exploited the advantages of a wing bending downward. The large lift near the lower final stroke position supports the reversal of the flap motion of the whole wing.

The angle of incidence of the hand wing, which has increased in the lower end position, becomes even larger in the course of its bending during upstroke. Due to the changing direction of its oncoming flow, however, this does not apply to its angles of attack. Along the hand wing will be occur a balance between positive lift close to the wrist and negative lift in the area near the wing tip, until is reached the maximum angle of its bending. This can then look in princiiple like in the example of a stretched hand wing (pleas see the diagram from the site Flight principle). Close to the wrist, however, its angle of attack is still increased. Together with the winglet effect of the hand wing, a larger lift can be held together in the arm wing and concentrated in the centre of the span, than without a strong bending.

When the arm wing reaches the upper end position, its angle of incidence at the wrist initially remains large due to the spring rod. Thus, without upstroke motion increases its angle of attack. After the upstroke motion, possibly with wing rotation to increase the lift in the middle of the span, so the lift is shifted a little to the outside. In the following waiting time, until the hand wing is stretched out, the arm wing has a very large lift near the wrist (compare with the animation of the swan). This is supported by the winglet or end plate effect of the still angled hand wing.

The large angle of attack at the wrist during the waiting period, naturally also applies to the hand wing. Its stroke torque upward is supported by the pressure equalisation of the adjacent large lift of the arm wing.

The advantage of the large lift in the upper stroke end position, is the missing wind turbine function with its negative thrust. Surely birds have such a possibility.

Subsequently also the hand wing reached the upper final stroke position respectively the extended wing position. There, because of the increased angle of attack at the wrist, grow its lift and thus its force upwards. As the spring rod now gives way, the angle of incidence on the wrist becomes smaller at the same time. Thereby the hand wing takes over parts of the lift from the arm wing. This continues because the angle of attack and so the lift of the hand wing increases further at the beginning of the downstroke.

The motion sequence of this wrist thus supports the process of the lift displacement between the arm and the hand wings. In addition, the motion sequence of the two wing sections reduces the mechanical stress on the spar and the drive mechanism in the final stroke positions. The moment of inertia of the single wing sections is clearly smaller than that of the whole wing. But a good possibility to control the curve of the model or to influence the twisting or rotating of the hand wing is still missing.